Three people have been confirmed dead in a deadly explosion that injured at least 140 others during a Boston Marathon on Monday.

Martin Richard, 8, of Dorchester was waiting at the finish line to hug his father when he was killed after two bombs, reportedly hidden in trash cans, were detonated within seconds of each other during the Patriots' Day bank holiday. A woman in her 20s was also among the three killed and the incident is being treated as the worst terrorist attack in the U.S. since 9/11, according to CNN.

(Photo: Reuters/Dan Lampariello)

Runners continue to run towards the finish line of the Boston Marathon as an explosion erupts near the finish line of the race in this photo exclusively licensed to Reuters by photographer Dan Lampariello after he took the photo in Boston, Massachusetts, April 15, 2013. Two simultaneous explosions ripped through the crowd at the finish line of the Boston Marathon on Monday, killing at least two people and injuring dozens on a day when tens of thousands of people pack the streets to watch the world famous race.

So far at least 18 of the injured are said to be in a critical condition and a number of victims may need amputations or major corrective surgery on maimed limbs. The youngest reported victim is said to be a two-year-old who is being treated for a head injury and the oldest injured victim is said to be in their 70s.

While the world continues to search for answers, on Monday it was reported that the the FBI is searching for a male suspect described as having dark skin, wearing black clothes and a black back-pack. He had reportedly tried to gain access into a restricted area during the annual marathon.

There are currently no suspects in custody.

President Barack Obama confirmed that the government is using various resources to carry out a thorough investigation.

"We still do not know who did this or why, and people shouldn't jump to conclusions before we have all the facts," President Obama said in a brief statement Monday evening.

"But make no mistake: We will get to the bottom of this," and whoever is responsible "will feel the full weight of justice," he added.

Eyewitnesses described the scene as being that of hysteria, horror and devastation.

"It felt like a huge cannon," a witness told CNN about one of the blasts.

Allan Panter, a doctor who was near the finish line waiting for his wife to finish the race, estimated that he was approximately 20 to 25 feet from the first blast. He said he treated victims on the street after the explosion.

"I saw at least six to seven people down next to me," he said. "They protected me from the blast. One lady expired. One gentleman lost both his (lower) limbs. Most of the injuries were lower extremities."